Journal of Scientific & Industrial Research Vo1.59, April 2000, pp 280-285 '

Causes and Remedial Measures of Pot-hole Subsidence due to Coal Mining

K B Singh

Central Mining Research Institute, Dhanbad 82600 1, India

Received: 17 May 1999; accepted: 10 January 2000

This paper discusses different causes and remedi al measures of pot-hole subsidence due to underground coal mining. Pot­hole subsidence is an abrupt local surface depression due to sudden collapse of overburden into the void created following the extraction of coal. Thi s can be hazardous to life and property as it occurs without any prior indication. Shallow depth of coal extraction, weak overburden, and geological discontinuities are the main causative factors . Rainfall, removal o f hydrostatic support, and earthquake aggravate the occurrence of pot-hole subsidence. Pot-holes subsidence can be controlled to some extent by using proper design of support and construction of walls to create a barrier around an area prone to pot-hole subsidence in board and pillar development. Back-filling and grouting can be used to stabilise unapproachable underground abandoned work­

ings.

Introduction

Subsidence is a time-dependent deformation on the surface topography, which is created by readjustment of the overburden above voids caused due to underground coal mining. Surface manifestation of underground coal extraction occurs in two forms, i. e., trough and pot-hole subsidence. Pot-hole subsidence is an abrupt depress ion of local ground surface, which occurs due to sudden col­lapse of the overburden into an underground void . Pot­holing phenomenon generally occurs following the fail­ure of mine roof which migrates through the overlying strata until the failure zone intercepts the unconsolidated overburden. These also occur due to formation of cavi­ti es in the overburden fol lowing the inflow of weak and weathered strata with water through fault plane. These cavities finally cave in and pot-hole appears on the sur­face. The pot-hole subsidence is very dangerous to lives and properties, as it does not give any prior warning. The geometry of the pot-holes is governed by its origin, the thickness and character of the overburden, the he ight of extraction, the state of stress existing in the roof, etc . The geometry of most of the pot-holes is circular or e l­liptical. The maximum diameter and depth of the pot­holes are, in general, less than 10m and 15 m, respec­tively. This paper reviews causes, mechanism, and re­medial measures of pot-hole subsidence along with ana lysed cases of29 pot-hole in Jamuna and Kotma Area of the South Eastern Coalfie lds Limited (SECL).

Causes of Pot-hole Subsidence

The main causes of pot-hole subs idence are a shal­low depth cover, weak overburden , and geo logica l

Table I---Mining depth for reported pot-ho le subsidence

S No Locat ion Maximum depth

(111)

Western Pennsylvania" 47.7

2 Hanna Area, Wyoming7 73.2

3 Beulan Area, North Dakota' 24.4

4 Sheridan Area,Wyoming' 77 .0

5 Illinois Coal Basin" Y 50.3

6 Colorado Springs Areall> 45 .7

7 Superior & Glenrock , Wyoming"' 11 30.5

8 Rock Springs, Wyoming l1 101.5

9 Handidhua & Dcu lbera collieries. Orissa 14 40.0

10 Scranton, USN ' 88 .0

II Indi ana, USN" 50.0

12 Humbersidc & Linco lnshire, UK 17 90.0

13 Eastern US Coalfi elds l" 30.0

14 Mithapurcolliery, West Bengali" 25 .0

15 Jamuna & Kotma Area, Mp lIl 43.0

SINGH : POT-HOLE SUBSIDENCE 281

60

40

.:0

: .. ,. -•

'0 20 30 40

Overburden thickness I mining holght ratio

Figure 1-Relationship between overburden thickness and

overburden thickness/mining height ratio

discontinuities . Rainfall , removal of hydrostatic support and earthquakes aggravate their occurrence. A brief de­scription to each causative factor is given below :

Shallow Depth

Shallow depth of working is a very important fac­tor, which controls the pot-holing phenomenon in dif­ferent part of the world. The depth of workings for the occurrence of pot-hole subsidence varies from 8 m to 101.5 m under different coal measure strata of the world (Table 1). Piggott and Eynon I have defined shallow depth as a maximum of I O-times the height of the original road­way excluding surface unconsolidated layers. A corre­lation was developed between pot-hole subsidence and overburden thickness (h) to mining he ight (m) rati02

.

When the ratio is less than 5, there is a strong possibility of the occurrence of pot-holes which decreases when the ratio is between 5 and II . It was reported that 90 per cent of total pot-ho les occurred when him rat io was 6 and the upper he ight of migration of void was 8- to 12-times the he ight of mini ng3. Cav ing he ight could reach to 7- to I O-times the mining height in the presence of the overly ing aquifer in room and pillar mining4. HuntS has developed a correlation between the depth of the work­in o-s and the so il thickness/ rock thickness ratio of the b

overburden in Illinois. He conc luded that soil thickness/ rock thickness ratio in the overburden fo r the pot-holes was O. I to 1.0.

Analysis of recorded pot-holes of the Jamuna and Kotma Area of the SECL in Shahdol di strict of Madhya

Scale. m o -----,

, , , , , Pot-hole

I • 1 , , , • • , Sub-soit to

); • I I I I X ); X

Weathered sandstone ---- - - ----- - -- --- - --- '

20

30

Soil and sand

Figure 2 --Pot-hole fonn ati on in weak overburden

Pradesh showed that pot-holes occurred between 16 to 63 m depth. The soil thickness/rock thickness ratio in the overburden for these pot-holes is 0.14 to 0.32. The ratio of overburden thickness to mining height for the most of the pot-holes of the Jamuna and Kotma Area is between 6 and 40 (Figure 1). A linear equation is developed between overburden thickness and overbur­den thickness/mining height ratio which is given be low:

y = 1.4004x + 9.554 ... ( I )

where, y = overburden thickness, In and x = overburden thickness/mining height ratio .

Index of determination for Eq . ( I ) is 0.9334. Equa­tion ( I ) can, also, be used by the coal mining industry for the prediction of pot-hole in advance under simil ar geo-mining condition .

Weak Overburden

Whenever an underground opening is created the strata immediate ly above the openin g become de­stressed. The opening re mains stable as long as the stresses do not exceed the strength of the roof rock. Over a period of time, roof stability may be jeopardi zed by the change in the stresses and strength of the roof rock . These changes may be due to groundwater inflow and creep up to great ex tent. The presence of water in the overburden also deteriorated the strength of the rock mass . Pot-holes occurred in the zones of weak overbur­den in Western Pennsy lvani a and Colorado Springs Ar­easo. I(). Caving of 10 to 15 m of a weak rock mass with an equal amount of sub-soil led to pot-hole formation (Fig­ure 2) at Handidhua and Deulbera co llie ri es in Orissa. The coarse grained sandstone in the overburden had 3 to

282 J SCI IND RES VOL 59 APRIL 2000

• MAL- S

Figure }--A part plan showing board and pillar development ,

fa ul t (F-F) and s u rf~ce locati on of a pot-hole at Meera Incline. Borehole locations are marked by MLA-l to MLA-6

4 kg/cm2 tensile strength l4 , These sandstone tended to flow in like sludge in saturated conditions . Two pot-holes occurred at Govinda colliery of Jamuna and Kotma Area, where saturated and weathered sandstones in the over­burden was friabl e and had 60 kg/cm2 compress ive

strength .

Geological Discontinuities

Geo log ical di scontinuities, particularl y c losely spaced joints and faults, play an important role in the formation of pot-holes . When a roof is formed from blocks bounded by joints, it may fail by shearing along planes of weakness when the vertical stress exceeds the shear res istance along the j oi nts. Extensive rock fractur­ing caused pot-holes in Superior, Wyomingl l. All the recorded pot-holes of the Jamuna and Kotma Area of the SECL occun'ed above developed and abandoned bord and pillar workings lying close to the incrop of the coal seams and Kewai Ri ver. A pot-hole at Meera Incl1l1e fo rmed fo ll owing the exposure of a faul t between 11 and 12 levels in the ma in dip (Figure 3) . Abnormally heavy water seepage was recorded through the fault plane along with sand and soil before formati on of the pot­hole. Handidhua and Deulbera collieries also witnessed imilar phenomenon along the fault planes.

Rai'~lall

RecharO'e of the overburden during the rainy sea-b

son increases the pore pressure, which can trigger roof fa ll. A relationship was establi shed between cumul at ive

precipitation and cumulative pot-hole development21 . It was found that pot-hole development reflected the quan­tity of precipitation experienced in the preceding 3 to 8 months. Esaki et al.22 have reported that the number of pot-holes increased rapidly during the rainy season in the Chikuho district in Japan . Rai nstorms caused pot­holes on the surface in St David, III inoisY

• Jamuna and Kotma Area witnessed the occurrence of II pot-holes during rainy season whereas occurrence period of 15 cases were not recorded . Remaining three cases of pot­holes occurred in dry period .

Removal of Hydrostatic Support

Removal of hydrostatic support to the roof in the old waterlogged workings is also a causative factor fo r the appearance of pot-hole20

. Old waterlogged workings in the Searsole seam have been standing on the small stooks at 8 to 22 m depth since 1928 at Mahabir colliery in Burdwan district of West Bengal in Ea tern Coalfie lds Limited (ECL). Removal of water from the upper o ld waterlogged workings of the Searsole earn into the lower acti ve workings of Niga seam caused removal of hydro­static support to the overlying strata. Thi resulted in the fai lure of the small stooks left in the Searsole seam and as many as 40 pot-holes appeared on the urface.

Earthquakes

Earthquakes can suddenly increase the number of pot-holes. The Tohoku district of Japan witnessed 15 to 20 pot-holes every year between 1947- 1987 except in 1987 when an earthquake of 7.4 magni tude caused 2 19 pot-holes22 • Underground blasting can a lso induce pot­holing phenomenon by way of loosening/opening of joints and faults. Thi s causes roof failure and inflow of washed out overburden materials which ultimately re­sult in the fo rmation of pot-hol es.

Mechanism of Pot-hole Subsidence

Pot-hole subsidence results from the intermit tent, sequential collapse, and unravel ing of underground mine roof in localized area, whereby caving migrates through the overly ing strata unti I the fracture zone intercepts the unconsolidated overburden . Generally the roof may fa il in two bas ic modes, i. e., tens ile and shearD. After the initi al tensile or shear failure of the roof, caving will continue to propagate upward until it is arrested by more competent roof layer or by bulking of the roof debris. The ex press ion of the feature at surface depends on the physico-mechanical prope rties of the unconsolidated overburden.

SINGH: POT-HOLE SUBSIDENCE 283

The mechanism of a pot-hole occurred at Meera Incline of Jamuna and Kotma Area of the SECL is dif­ferent. The overburden comprises saturated (water bear­ing) sandstone and alluvium. The upper portion of the sandstone lying just below the alluvium is weathered and fri able. When the face advanced 10m ahead of the Main Dip-II Leve l junction in the Upper Kotma (top section) seam, a fault wa s intersected (Figure 3). Therafter, heavy inflow of water with sand and soi I was noticed along the fault plane. The magnitude of water inflow increased to 9000 IImin . The inflow of water was measured by installing V-Notch across the water flow close to exposed fault plane. The sand and soil were eroded from the weathered , weak and friabl e sandstone which resulted in the formation of the cavities in the overburden (Figure 4). Thus, a pot-hole appeared on the surface within 7d after the intersection of the fault pl ane in the heading, following the collapse of the cavities.

Preventive Measures

It has already been mentioned that pot-holing phe­nomenon creates very serious problems due to the ab­sence of any prior indications . Therefore, it is necessary to know the preventive measures to avoid thi s phenom­enon . The important preventive measures include: de­signing of proper stiffness of support , constlUcton of wall s, grouting of voids, back fillin g, filling of cracks, etc . A brief description to each preventive measure is di scussed below:

Support

Designing proper supports should support the area prone to the occurrence of pot-holes in the board and pillar workings. It is advised to des ign support based on CMRI geo-mechanical classification ofIndian coal mea­sures24 . Thi s classification is based on stati stical study on exposed roof with respect to layer thickness, struc­tural discontinuities, weatherability, compress ive strength and ground water seepage. These factors allo­cated 30, 25, 20, 15, and 10 per cent ratings on the basis of stati stical analysis of Indi an coal measures . The indi­vidual rating is added to know Rock Mass Ratin gs (RMR). The Indian coal mine roof is divided into five classes. The roof support is.designed based on the above classification following the computation of rock pres­sure as given in Eq. (2).

Pr = B.Y.rock load factor,

Pr = B. Y. ( 1.7-0.037 RMR + 0.0002 RMR2), ... (2)

where, P = rock load, tim" , B = roof span , fII , and y = r

rock density, t/m3•

Alluvium t-:--------

Figure 4-A schemati c diagram showing mechani sm of pot-ho le formation at Meera Incline

Construction of Walls

When a collapsed roof and inflow of sand and so il with water along a fault pose a significant risk for pot­holes at the surface in the deve lopment headings, suit­able walls can be constlUcted to form a barrier around the collapsed area and zone of inflow of sand and soi l with water. Underground volume of void is reduced by erecting the suitable walls. Inflow of eroded soil and sand automatically be stops after filling of this reduced void and thus, void formation in the overburden can be avoided. Thi s has been highly successful in the pot-hole prone areas of British coalfields 17 and Jamuna and Kotma area of the SECUo.

Grouting of Voids

This approach can be applied as a local area stabilisation technique to support buildings, hi ghway, school , religious buildings, etc. There are two commonly used grouting techniques 'x. 25 . One utilizes a sand grout mixture, which is injected into IUbblized zones of roof failure and small void to support the roof and e liminate the available space for continued downward movement of the overburden (Figure 5). Another approach can be used in the areas of large voids. This employs gravel­grouted columns to provide direct support to the mine roof (Figure 6) . Several pot-holes occurred over aban­doned bord and pillar workings in Glenrock, Wyoming which created danger to highway, res idential and com­mercial complexes. These surface stlUctures were suc­cessfully stabilized by using 6670 m3 grout mixture .

Back-jllling

Normally hydraulic back-filling technique is used to stabilize abandoned workings on a regional basis. Sand, fly ash , crushed rock, and coal mine refuse are used as a fill materials . The stiffer the fill the sooner the

284 J SCI IND RES VOL 59 APRIL 2000

Ground surface

G<out injection ,.. --SmaU voids Cased as necessary

"""""'~==jl~~mI;;n;im;:um:::. Tdlameter

roo,:...'_~~

111!

Note: withdraw casing and grout injection pipe a!: grouting operation proceeds Low pressure grouting recommended

Figure 5 ---Rubble and small void grouting25

passive resistance will be mobilized and limit defonna­tion and restrict the progression of failure. The entire city of Rock Springs spreading over 13 .5 ha, Wyoming lied over abandoned underground coal mine25 . High pil­lar ex traction and robbing caused 78 subsidence events wherein large majority of them were pot-holes. Hydrau­lic back-filling to abandoned underground workings us­ing abandoned surface coal mine spoi ls saved the entire city. The total quantity of back-fill material was 200,000 tones and the cost of back-filling was US $ 55,403/ha.

Filling of Cracks

Soil and Sand - Cement mixture above alluvium and rocks, must fill in cracks fonned due to ground move­ments, respect ively. However, if the cracks have devel­oped in river or ri vulet beds, they should be in-filled by concrete. Thi s restricts the penetration of su'rface water through cracks or faults and thereby reduces the erosion of the overburden, thus reducing the possibility of pot­hole fonnation.

Conclusions

Pot-hole subsidence caused due to underground coal mining poses serious threat to life and property due to the lack of any surface indications prior to their for­mations. They occurred up to 63 m depth in Jamuna and Katma area of the SECL in India. The pot-holes occurred where the so il thickness to rock thickness ratio in the overburden was 0.14 to 0.32. A linear equation was de­veloped between overburden thickness and overburden thickness/mining height ratio. Fault created a direct path between the underground workings and weak weathered

Ground surface

Grout inJection pIpe

Cased as necessory. minimum 6" diameter

------

Mine floor

Note: 'place gravel after Inserting injection pipe but prior to injecting grout

withdraw Injection pipe as grouting from bottom of void to top

Figure 6--Gravel and grout column 25

water bearing overburden in which cavity is created . This cavity caved in and thus pot-hole appeared on the sur­face . Pot-holing phenomenon can be controlled to some extent by using proper support and providing a barrier to restrict inflow of weathered material by erecting com­petent walls in the development worki ngs. Grouting and back-filling can be used to stabilize overburden lying above unapproachable abandoned workings.

Acknowledgement

Financial support provided by the Chairman-cum­Managing Director, SECL to undertake this study is ac­knowledged.

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SINGH : POT-HOLE SUBSIDENCE 285

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